The invention relates to a neutral setting apparatus for adjustable hydraulic machines, in particular the adjustment of the neutral position of a servo valve. The invention relates in particular to hydrostatic adjustment devices of hydraulic machines, in which both the volumetric delivery and the delivery direction can be set. The invention relates specifically to a servo valve with a control piston for controlling a servo piston which, in turn, sets the volumetric delivery and the delivery direction of the adjustable hydraulic machine. The present invention relates in particular to servo adjustment devices with mechanically adjustable control pistons, wherein the forces necessary for this can be applied mechanically, electromagnetically, pneumatically or hydraulically.
Hydraulic servo valves of very different design are used for adjusting the volumetric delivery of hydraulic pumps. In this case, a servo valve of this type is used to act upon a servo piston with hydraulic fluid or hydraulic pressure in a controlled manner such that the servo piston adjusts the actual adjustment device of the hydraulic machine, for example the swash plate of an axial piston machine. The invention can be used for servo valves of this type. Examples of further fields of use are the control systems of radial piston machines, the eccentricity of which can be set, or inclined-axle machines, the performance or else delivery direction of which can be changed by deflection of a yoke. The servo pistons which act on the adjustment devices of the hydraulic machines are customarily centred in the zero position thereof via springs, as a result of which, when pressure conditions are equalized, for example at a double-sided servo piston, the delivery flow of the hydraulic machine is zero.
The volumetric delivery of zero corresponds to the shutdown of the machine, i.e. power is neither received nor output by the hydraulic machine. This shutdown of the machine is of importance in terms of safety and therefore has to be exactly predefinable by the servo valve device. Via the control edges thereof, the control piston, which is responsible for the pressures applied to the servo piston, in the control valve controls the respective hydraulic pressures to the servo piston or the servo pistons, and therefore the hydraulic neutral position of the control piston in the control cylinder, i.e. the position of said control piston for the shutdown of the hydraulic machine, necessarily has to be exactly settable.
In practice, however, the diameter and the control edges of the control piston and the diameter and the corresponding control edges of the control cylinder are affected by tolerances during production, and therefore the neutral position of the control piston in the control cylinder generally deviates from the structurally predefined central position or the geometrically centred position. If it is therefore intended to centre the control piston of a servo valve in the control cylinder of the servo valve purely geometrically, an asymmetrical pressurization of a, for example, two-sided servo piston cannot be ruled out. This causes displacement of the servo piston, and the adjustable hydraulic machine would be outside the zero position and a shutdown of the machine would not be achievable. A mechanism for the neutral setting is therefore necessary in order to compensate for the position error, caused by manufacturing tolerances, of the control piston in the control cylinder so that, in the hydraulic neutral position of the control piston, the hydraulic machine enables the zero position of the servo piston and therefore a shutdown of the machine can be achieved.
It is ensured by a neutral setting adjustment that, in the event of an indicated position of the servo system, in which the hydraulic machine does not generate a delivery flow, an activating signal counteracting said state is not generated in the servo valve. Failing this, the position of the control piston in the servo valve does not match the position of the servo piston in the adjustment device for the hydraulic machine. In such a case, a shutdown of the machine can never be achieved, since one of the two pistons is always outside the hydraulic centre. A neutral adjustment for the servo valve therefore has the task of centering the control piston in the servo valve in the control cylinder, in the event of shutdown of the machine, such that suitable control edge overlaps with corresponding control edge intervals do not result in any fluid flows or servo pressures which would bring the servo piston of the adjustable hydraulic machine out of balance counter to the spring forces.
As a rule, mechanically actuated servo valves are controlled via Bowden cables or linkages in order to regulate the volumetric delivery of the adjustable hydraulic machine in both delivery directions. Said mechanical activation is intended to react as identically as possible for both delivery directions. This gives rise to the requirement for a symmetrical, but respectively small, dead band of identical size in both delivery directions, within which the pump does not generate any volumetric delivery. At the same time, the maximum volumetric deliveries of the two delivery directions of the adjustable hydraulic machine are also intended to be achieved with an input signal of identical size. Specifically for mechanical adjustments, this means that the deflection of the control apparatus in one direction is intended to be of precisely the same magnitude as in the other direction, so that the discharge capacity which is generated by the adjustable hydraulic machine or is received by the adjustable hydraulic machine is of identical size for both delivery directions. In particular, movement forwards and backwards or pivoting to the left and right, which is intended to take place with the same power in each case, is conceivable here. In a known embodiment for a mechanical drive of a servo valve by means of, for example, a Bowden cable or a linkage, a torque acts on an input shaft on the servo valve in order to rotate said input shaft in the one or other direction of rotation. For various reasons, in particular for safety reasons and for ease of operation reasons, said input shaft must always automatically endeavour to return into the set neutral position thereof. Specifically, the machine operator expects that, after letting go of the deflected control lever, the latter will automatically pivot back again into the neutral position. This can be achieved, for example, by a permanently acting spring force in the servo valve.
In a known embodiment of a neutral setting mechanism of this type, the input shaft, which can be rotated mechanically in two directions, has a flattened portion on which a guided sliding part loaded by spring force acts. The sliding part has a likewise planar face on the contact face between the flattened portion and the end face of the sliding part, as a result of which, when the input shaft rotates out of the neutral extra-axial force applied by the spring force, which acts on the sliding part, generates a resetting torque on the input shaft. Said resetting torque attempts to move the input shaft back again into the neutral position thereof, in which the two faces, i.e. the flat end face of the sliding part and the flat flattened portion on the input shaft rest evenly or flat on each other. In said flat, sheet-like contact, the spring force acts directly in the direction of the axis of the input shaft, and therefore torque is not generated by the spring force. By means of the sheet-like contact of the sliding part with the flattened portion of the input shaft, the sliding part is displaced away from the axis of the input shaft, specifically always in the same direction counter to the spring, irrespective of the direction of rotation of the shaft. As a result, a torque acting in each case in the one or other direction is generated when the input shaft is deflected. If the deflecting torque at the input shaft is smaller than the torque which is generated by the displaced sliding part via the flattened portion on the input shaft, the input shaft automatically rotates back into the neutral position thereof. For the setting/adjustment of the neutral position of the servo valve, the known embodiment proposes displacing the connecting lever, which connects the input shaft to the control piston, relative to the position feedback device via an eccentric. The control piston in the control cylinder therefore adopts a neutral position for the servo adjustment, irrespective of the position of the input shaft. The signal which is passed on by the position feedback means regarding the position of the adjustment device of the hydraulic machine is thus matched to the position of the control piston.
However, since the neutral position of the servo piston in the servo cylinder does not correspond in most situations to the geometrical centre, the deflectability of the control piston in one direction is therefore smaller than the deflectability of the control piston in the other direction, which leads to different volumetric deliveries in the two delivery directions. In the end result, this leads to an asymmetrically adjustable hydraulic machine. The greater the position error to be corrected between the servo valve, i.e. the control piston in the control cylinder, and the involved levers and the involved guides, the greater is the asymmetry between the input signal and the delivery flows in the two delivery directions. Therefore, in this case, different flow rates occur in the one or other delivery direction, since the maximum volumetric deliveries of the hydraulic fluid which can be achieved in the one delivery direction or in the other delivery direction differ.
The invention is therefore based on the object of providing an apparatus for the neutral setting of servo valves for adjustable hydraulic machines, which apparatus ensures the neutral position of the control apparatus when a machine is at a standstill and, furthermore, guarantees symmetry of the volumetric deliveries in both delivery directions. Furthermore, it is an object of the invention to specify a setting mechanism for the neutral position of a servo valve, the setting mechanism managing with just a few components and the construction thereof being simple, robust and cost-effective.
The object according to the invention is achieved with a neutral setting apparatus according to Claim 1, wherein preferred embodiments are specified in the claims dependent on Claim 1.